Surface charge enabled nanoporous semi-permeable membrane for desalination

ABSTRACT

A filter includes a membrane having a plurality of nanochannels formed therein. A first surface charge material is deposited on an end portion of the nanochannels. The first surface charge material includes a surface charge to electrostatically influence ions in an electrolytic solution such that the nanochannels reflect ions back into the electrolytic solution while passing a fluid of the electrolytic solution. Methods for making and using the filter are also provided.

RELATED APPLICATION INFORMATION

This application is a Divisional application of co-pending U.S. patentapplication Ser. No. 12/607,258 filed on Oct. 28, 2009, incorporatedherein by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to semi-permeable membranes and moreparticularly to a semi-permeable nanoporous membrane and methods formaking and using the same for desalination and other processes.

2. Description of the Related Art

Water desalination may be thought of in terms of two approaches. The twobasic approaches for water desalination include reverse osmosis anddistillation. The distillation approach requires converting fluid waterto the vapor phase and condensing water from the vapor. This approach isfairly high cost and requires significant energy usage. The reverseosmosis approach uses pressure on a salinated liquid to force watermolecules through a semi-permeable membrane. This approach has arelativity low rate of energy consumption.

The specific (per unit of produced potable water) energy of desalinationusing reverse osmosis has been reduced from over 10 k Wh/m³ in the1980's to below 4 k Wh/m³, approaching the theoretical minimum requiredenergy of 0.7 k Wh/m³. To improve the state of art of the reverseosmosis approach, new membranes, with a uniform pore distribution and amore permeable separation layer can potentially maintain or improve saltrejection while increasing the flux in the reverse osmosis method. Suchimprovements have not yet been developed in the conventional art.

SUMMARY

A filter includes a membrane having a plurality of nanochannels formedtherein. A first surface charge material is deposited on an end portionof the nanochannels. The first surface charge material includes asurface charge to electrostatically influence ions in an electrolyticsolution such that the nanochannels reflect ions back into theelectrolytic solution while passing a fluid of the electrolyticsolution. Methods for making and using the filter are also provided.

A filter system includes a first volume configured to receive anelectrolytic solution at a pressure, and a second volume separated fromthe first volume by a membrane having a plurality of nanochannels formedtherein. The nanochannels include a first surface charge materialdeposited on at least one end portion of the nanochannels. The firstsurface charge material includes a surface charge to electrostaticallyinfluence ions in an electrolytic solution such that the nanochannelsreflect ions back into the electrolytic solution while passing a fluidof the electrolytic solution.

A method for making a filter includes forming a plurality ofnanochannels in a membrane; and depositing a first surface chargematerial on at least one end portion of the nanochannels. The firstsurface charge material includes a surface charge to electrostaticallyinfluence ions in an electrolytic solution such that the nanochannelsreflect ions back into the electrolytic solution while passing a fluidof the electrolytic solution.

A method for filtering an electrolytic solution includes filling a firstvolume with an electrolytic solution, applying a pressure below athreshold value to the electrolytic solution in the first volume, andpassing a fluid of the electrolytic solution into a second volumeseparated from the first volume by a membrane. The membrane has aplurality of nanochannels formed therein. The nanochannels include afirst surface charge material deposited on at least one end portion ofthe nanochannels. The first surface charge material includes a surfacecharge to electrostatically influence ions in an electrolytic solutionsuch that the nanochannels reflect ions back into the electrolyticsolution while passing the fluid of the electrolytic solution.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view taken along a longitudinal axis ofnanotubes or nanochannels through a membrane showing the nanochannelsforming a double electrical layer in accordance with one illustrativeembodiment;

FIG. 2 is a top view of a membrane showing nanochannels and locating asection A-A in accordance with one illustrative embodiment;

FIG. 3 is a cross-sectional view taken along section A-A of FIG. 2showing the formation of nanotubes or nanochannels through the membranein accordance with one illustrative embodiment;

FIG. 4 is a cross-sectional view showing a surface charge layer formedover one end portion of the nanotubes or nanochannels in accordance withone illustrative embodiment;

FIG. 5 is a cross-sectional view showing two surface charge layersformed over two end portions of the nanotubes or nanochannels inaccordance with another illustrative embodiment;

FIG. 6 is a block diagram illustrative depicting a desalination systemin accordance with an illustrative embodiment;

FIG. 7 is a flow diagram showing a method for making a filter inaccordance with the present principles; and

FIG. 8 is a flow diagram showing a method for using a filter inaccordance with the present principles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, a new membrane is described,which utilizes a surface charge of nanopores and/or nanochannels. In oneembodiment, for reverse osmosis, a high salt rejection is achieved whilesimultaneously maintaining high flux. In one embodiment, a nanoscalefilter includes arrays of parallel nanopores or channels which areformed into a membrane material. The surface of the nanopore or channelis configured with a material, which has a high negative (or positive)surface charge when exposed to an electrolyte. This effect blocks thetransport of ions through the channel, and is effectively an ion filter.

It is to be understood that the present invention will be described interms of a non-limiting semi-permeable membrane formed from a materialincluding aluminum; however, other structures, membrane materials,coating materials, process features and steps may be varied within thescope of the present invention. The membrane may be formed in sheets andcut to size or may be formed in or included with pre-sized panels.

In particularly useful embodiments, the membrane is employed fordesalination of water. However, other physical or chemical processes mayemploy the present principles.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, across-sectional view of ananoscale filter 10 in accordance with the present principles isillustratively shown. Filter 10 includes arrays 12 of parallel nanoporesor channels 14 which are formed into a membrane material 16. A surfaceof the nanopore or channel 14 is configured with a material 18, whichhas a high negative surface charge (or high positive surface chargedepending on the application) when exposed to an electrolyte 20, such assalt water. In one embodiment, a deposition material 17 may be depositedto narrow the openings in the nanotubes 14. The material 17 ispreferably deposited using, e.g., a chemical vapor deposition (CVD) or aphysical vapor deposition (PVD). Material 17 is needed only to adjustthe size of the channels 14 and may be omitted if channels 14 are ofsufficient size using a layer of material 18.

In one embodiment, the material 18 may include, for example, titaniumdioxide or silicon dioxide. In the electrolyte 20, a negative surfacecharge on surfaces 22 will attract positive ions (counter-ions) in theelectrolyte 20, which forms an electrical double layer 26 (in a vicinityof layer 18). A positive surface charge on surface 22 will attractnegative ions (anions) in the electrolyte 20, which forms the electricaldouble layer 26. The double layer 26 includes the surface charge of thenanochannels 14 and the electrolytically responsive ions thereto.

In one embodiment, a thickness t of the electrical double layer willdepend on a charge density of the electrolyte 20, and is around 1 nmwhen electrolyte densities are in the range of 1.0 molarity (M). As anexample, if the thickness of this electrical double layer is about ½ ofa pore or channel diameter, the counter-ion regions from one side of thepore or channel will merge with the region from the opposite site,forming a region 30 across the pore or channel diameter which containsonly positive charge, since the negative charge is repelled by thenegative surface charge 22 of the nanopore or nanochannel surface. Thiseffect blocks the transport of negative ions through the channel, and iseffectively a negative ion filter. The opposite polarity effect shouldoccur for a surface which is positively charged, such as, with a siliconnitride surface.

Referring again to FIG. 1, for sea water, the charge density of ions issuch that the thickness of the electrical double layer 26 is on theorder of 1 nm, so a nanopore or nanochannel of diameter less than 3 nmwould be needed. Lower concentration salt water would allow the usage ofa larger diameter nanopore or channel. One criteria being a ratio ofapproximately 2:1 for the diameter of the pore or channel compared tothe thickness of electrical double layer in the salt-water(electrolyte).

Referring to FIG. 2, a top view looking into an array 12 of nanopores ornanochannels 14 is illustratively shown. A device or membrane 10 can befabricated with parallel nanopores or nanochannels 14 on the scale of 3nm-20 nm in diameter. Other sizes are also possible and may be employeddepending on the application. One approach to accomplish this would beto form nanochannels 14 in aluminum foils (16) using anodic aluminumoxide (AAO) (See e.g., O. Jessensky et al., “Self-organized formation ofhexagonal pore arrays in anodic alumina” Appl. Phys. Lett, 72, (1998)p1173, also G. Sklar et al, “Pulsed deposition into AAO templates forCVD growth of carbon nanotube arrays”, Nanotechnology, 16 (2005)1265-1271). This process forms high aspect ratio, parallel channels intoAl by an anodic oxidation process.

Referring to FIG. 3, a cross-sectional view taken at section A-A of FIG.2 is illustratively shown. Nanotubes or channels 14 are formed in aparallel manner through material 16.

Referring to FIG. 4, once the array 12 of nanochannels 14 is formed, thesurface can be conditioned by depositing material 17, such as, SiO₂ orother suitable material preferably using a chemical vapor deposition(CVD) technology or sputter deposition (known as Physical VaporDeposition or PVD). The goal here is to close off the top aperture ofthe nanochannel 14. Both PVD and CVD are not very conformal depositiontechnologies; they tend to clog the channel at the opening of the hole.By controlling the thickness of this deposition, the opening of thenanochannel can be controllably shrunk to any dimension. The materialused for this can have an impact on the surface charge, or else it canbe covered over with a thin layer of TiO₂ or other charge material 18(FIG. 1 or FIG. 5), using, e.g., atomic layer deposition (ALD), whichforms a surface with a high negative surface charge. Alternately,material 18 may include a material for a positively charged surface. Thematerial 18 may be deposited on one end of the nanochannel 14.

Referring to FIG. 5, if desired, a second layer 52 of material could bedeposited on the far end of the nanochannel 14. For example, a first endportion 55 includes a negatively charged surface while a second endportion 56 includes a positively charged surface. In one embodiment,silicon nitride may be deposited by a plasma-enhanced CVD or a reactivesputter deposition process to form a surface with a positive surfacecharge. The opening of each end of the nanochannel 14 can be adjusted tothe 3-10 nm range by controlling both the initial diameter of thenanochannel 14 in AAO (e.g., 20 to 200 nm) and the subsequent depositionof a filler material 17, or the surface coating material 18 (and/or 52).It is only necessary for this coating 18 (or 52) to occur at the veryends of the channel, since that is where the electrostatic filtering ofions will take place.

Referring to FIG. 6, a desalination device 100 includes a membrane orfilter 102 having a large array of parallel nanochannels. The membrane102 may be configured on a grid, mesh or other structural member 110 forstrength. The nanochannel arrays of membrane 102 separate a fluid volumein a first reservoir or container 104, e.g., containing a salt watersolution from a second volume 106 in which the sodium and chlorine ionsdo not penetrate, hence forming desalinated water. The application ofpressure P to the salinated side would increase the permeation of watermolecules through the nanopore/nanochannel array of the membrane 102 upto a point. At a high enough pressure P, the flow of water through theapertures would exceed the ability of the surface charges to reflect theions, and hence the ionic filtering capability would break down. Thevolume 104 in FIG. 6 could have flow into and out of the volume 104. Itshould be noted that the flow of water into volume 104 should be wellmixed to prevent a significant build up of ionic charge in the volume104. Salinity of water in volume 104 should be regulated over timebecause as the desalinated water leaves, the salt stays behind. Thiswill increase the concentration until eventually the surface charge canno longer block the nanochannels of membrane 102. It is thereforepreferably to have an open system where water is replenished to counterbuild up in ionic concentration. A mixer or other perturbation device120 may be useful to stir the water in volume 104.

A critical flow could be calculated in a manner using Child's Law, inwhich the charges within the aperture shield the upstream charges fromthe applied fields, and hence at that point (and flow), ionic filteringwould cease. A pressure regulator device 112 may be employed to maintainthe pressure P at or below this critical pressure value to ensure properfunctioning of the desalination system 100. Alternately, a container maybe configured to provide a working pressure P using the height of awater column in the fluid volume or container 104 or by other means.

In one embodiment, the arrays of parallel nanopores or channels in themembrane 102 are coated on one end with a material to create a negativesurface charge (e.g., titanium dioxide or silicon dioxide). In anotherembodiment, the other end may be coated with a material to create apositive surface charge (e.g., silicon nitride). Note coating one end,for example, with a negative (or positive) surface charge material willwork for both types of ions. The high surface charges in an electrolyteattract or repel ions and form an electrical double layer at one or moreend to repel ions. The thickness of this electrical double layer may beabout 1 nm for electrolyte densities in the 1.0 M range (e.g., seawater). For a thickness of this electrical double layer of half the porediameter, the transport of ions through the channel is blocked and anion filter is formed.

The embodiment depicted in FIG. 6 may be extended to include a pluralityof membranes in series to further refine the filtration. In oneembodiment, different filtration stages may be employed wherein at eachstage pressure is controlled to ensure that each stage is performingefficiently. This may include increasing or decreasing the intermediatepressures of the fluid at each stage.

A sequential embodiment may include additional membranes 102′ and stages116 for filtering at different dimensions. For example, sea water in thefirst volume 104 could go through an intermediate filter (102) first,which would block some but not all of the ions. A second-stage filter102′ could then be used with has different diameters for nanochannelsand hence blocks a different concentration.

While desalination of water has been described as an illustrativeexample, other fluids may be filtered in accordance with the presentprinciples. In addition, different materials and combinations ofmaterials may be employed to provide electrostatic filtering.Advantageously, a desalination system can be provided that is passive(does not require a power source) and may be employed as an inexpensivedesalination system, an emergency desalination system (e.g., on liferafts) etc.

Referring to FIG. 7, a flow diagram is shown for an illustrative methodfor making a filter in accordance with the present principles. In block202, a plurality of nanochannels is formed in a membrane. The formingmay include using anodic aluminum oxide to form the nanochannels in analuminum membrane. The nanochannels may include a diameter of betweenabout 3 nm and 200 nm.

In block 203, a material may be deposited to adjust the apertures of thenanochannels. The material is preferably deposited using a CVD or PVDprocess to incrementally narrow the channel openings to provide anappropriate aperture dimension.

In block 204, a surface charge material is deposited on at least one endportion of the nanochannels. The surface charge material includes asurface charge to electrostatically influence ions in an electrolyticsolution such that the nanochannels reflect ions back into theelectrolytic solution while passing a fluid of the electrolyticsolution. In block 206, a second surface charge material may bedeposited on a second end portion of the nanochannels. The secondmaterial includes a surface charge to electrostatically influence ionsin an electrolytic solution such that the nanochannels reflect ions backinto the electrolytic solution while passing the fluid of theelectrolytic solution.

In an alternative embodiment, depositing the first surface chargematerial may include depositing the first surface charge material onfirst locations on a first end portion of the nanochannels, anddepositing a second surface charge material on second locations on thefirst end portion of the nanochannels, such that the first and secondsurface charge materials provide opposite polarities for the surfacecharge. Different polarities may exist on a same side of the membrane byusing resist masks or other large scale integration techniques. Inaddition, one configuration may include different surface polarities ona same side of the membrane and different polarites on the opposite sideof the membrane. Different patterns and different configurations may beprovided. For example, the first surface charge material may include amaterial having a negative surface charge to repel negative ions and apositive surface charge to repel positive ions on an opposite side ofthe membrane or on the same side of the membrane, etc. It should beunderstood that the illustrative examples as described here may becombined in any manner and provide many useful configurations inaccordance with the present principles.

Referring to FIG. 8, a flow diagram is shown for an illustrative methodfor using a filter in accordance with the present principles. In block250, a first volume is filled with an electrolytic solution. This volumeis preferably mixed or constantly replenished. In block 252, a pressureis applied to the electrolytic solution in the first volume that isbelow a pressure threshold value. In block 254, a fluid of theelectrolytic solution is passed into a second volume separated from thefirst volume by a membrane. The membrane has a plurality of nanochannelsformed therein as described above. The fluid in the second volume isdesalinated or partially desalinated. Additional stage may be added.

Having described preferred embodiments of a surface charge enablednanoporous semi-permeable membrane for desalination (which are intendedto be illustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A method for making a filter, comprising: forming a plurality ofnanochannels in a membrane; and depositing a first surface chargematerial on at least one end portion of the nanochannels, the firstsurface charge material including a surface charge to electrostaticallyinfluence ions in an electrolytic solution such that the nanochannelsreflect ions back into the electrolytic solution while passing a fluidof the electrolytic solution.
 2. The method as recited in claim 1,wherein forming includes using anodic aluminum oxide to form thenanochannels in an aluminum membrane.
 3. The method as recited in claim1, wherein forming includes forming the nanochannels to include adiameter of between about 3 nm and 200 nm.
 4. The method as recited inclaim 1, wherein the first surface charge material includes a materialhaving at least one of a negative surface charge to repel negative ionsand a positive surface charge to repel positive ions.
 5. The method asrecited in claim 1, further comprising depositing a second surfacecharge material on a second end portion of the nanochannels, the secondsurface charge material including a surface charge to electrostaticallyinfluence ions in an electrolytic solution such that the nanochannelsreflect ions back into the electrolytic solution while passing the fluidof the electrolytic solution.
 6. The method as recited in claim 1,further comprising adjusting an aperture dimension of the nanochannelsusing a deposition process.
 7. The method as recited in claim 6, whereinthe aperture dimension is controlled by controlling a deposition rate ofthe deposited material.
 8. The method as recited in claim 1, wherein thenanochannels are sized to include a diameter of about double a size ofan electrical double layer formed by the electrostatically influencedions in an electrolytic solution.
 9. The method as recited in claim 1,wherein the first surface charge material is deposited on a filler layerformed over the at least one end portion of the nanochannels, the fillermaterial adjusting an initial diameter of the nanochannels.
 10. A methodfor filtering an electrolytic solution, comprising: filling a firstvolume with an electrolytic solution; applying a pressure below athreshold value to the electrolytic solution in the first volume; andpassing a fluid of the electrolytic solution into a second volumeseparated from the first volume by a membrane, the membrane having aplurality of nanochannels formed therein, the nanochannels including afirst surface charge material deposited on at least one end portion ofthe nanochannels, the first surface charge material including a surfacecharge to electrostatically influence ions in an electrolytic solutionsuch that the nanochannels reflect ions back into the electrolyticsolution while passing the fluid of the electrolytic solution.
 11. Themethod as recited in claim 10, wherein passing a fluid includes passingthe fluid through a plurality of membranes.
 12. The method as recited inclaim 11, wherein each of the plurality of membranes includes adifferent aperture dimension for the nanochannels.
 13. The method asrecited in claim 10, further comprising regulating ion concentration inthe first volume.
 14. The method as recited in claim 10, furthercomprising regulating a pressure of the first volume.
 15. The method asrecited in claim 10, further comprising proving a support structureconfigured to support the membrane against the pressure.
 16. The methodas recited in claim 10, further comprising mixing the fluid in the firstvolume.
 17. The method as recited in claim 10, wherein the first surfacecharge material includes a material having a negative surface charge torepel negative ions.
 18. The method as recited in claim 10, wherein thefirst surface charge material includes a material having a positivesurface charge to repel positive ions.
 19. The method as recited inclaim 10, further comprising providing a second surface charge materialdeposited on a second end portion of the nanochannels, the secondsurface charge material including a surface charge to electrostaticallyinfluence ions in an electrolytic solution such that the nanochannelsreflect ions back into the electrolytic solution while passing the fluidof the electrolytic solution.
 20. The method as recited in claim 10,wherein the nanochannels include a diameter of about double a size of anelectrical double layer formed by the electrostatically influenced ionsin an electrolytic solution.